Discrete multi-tone transmission (DMT) may be used to transmit data through a link. For example, DMT transmission is used in asymmetric digital subscriber line (ADSL) systems, where its use is specified by the G.992 standard published by the International Telecommunications Union (ITU).
In the transmission system specified by this standard, a number of input bits are divided between a plurality of discrete “tones”, i.e. frequencies. Depending on the exact properties of the link between the transmitter and the receiver, and various constraints, in particular as to output power, some tones are capable of transmitting a larger number of bits than other tones. Accordingly, some of the tones are generally allocated more bits than other tones. A symbol is formed containing a stream of samples calculated by an inverse discrete Fourier transform of constellation points representing the bits of data on each tone. The stream of samples is then transmitted through a channel to a receiver. A sequence of symbols is transmitted sequentially to transmit a data stream.
A difficulty exists in that the channel normally has an impulse response that is not negligible. Thus, when several symbols are transmitted sequentially down the channel interference between the symbols may occur.
The effect of the channel can be considered to be a finite impulse response filter with L taps.
One way of dealing with the effects of the channel is to provide additional samples, known as a cyclic extension, at the beginning of each symbol. The last M samples of each symbol are appended in front of the symbol. This approach can require very long cyclic extensions and thus a prohibitive loss of efficiency.
To address this difficulty, a filter may be provided on the input of the receiver to reduce the effects of intersymbol interference in the channel. Such approaches are disclosed in U.S. Pat. No. 5,870,432, and the paper by N. Al-Dhahir and J. M. Cioffi “Efficiently Computed Reduced-Parameter Input-Aided MMSE Equalizers for ML Detection: A unified Approach”, IEEE Transactions on Information Theory, vol. 42, No. 3, May 1996. Both of these documents are incorporated herein by reference in their entirety.
In this approach, a Finite Impulse Response (FIR) filter, called Transversal Equalizer (TEQ), may be inserted at the input of the receiver to correct for the impulse response of the channel and output a signal having reduced cyclic prefix length compared with that of the channel. The algorithm computes the TEQ and finds the FIR coefficients such that the convolution of the Channel Impulse Response (CIR) with the TEQ, called CIReq matches in some sense a Target Impulse Response (TIR) with a length of M+1 taps, where M is the number of samples of the cyclic prefix. As presented in the prior art, the principle of this algorithm is to fix the length of the cyclic prefix and then to shorten the CIR to M+1 taps using the TEQ. As the CIR equation is often in fact longer than M+1 samples, there is a residual error causing intersymbol and intercarrier interference (ISI/ICI) between DMT symbols and thus limiting the data rate over the channel.
A method for frequency interpolation of a DMT system is presented in U.S. Pat. No. 6,088,386 entitled “Transmitter with phase rotor, modulator and demodulator, communication system and method performed thereby”, assigned to Alcatel. The contents of this patent are hereby incorporated in their entirety by reference. When a pair of modems are connected together their internal clocks will in general have slightly different frequencies, and accordingly the transmitted and received symbols will not be aligned. The method described in this Alcatel patent aligns the data symbols transmitted from one of the pair of modems with the symbols transmitted from the other. The method achieves the alignment by including in a modem bit addition and deletion means which add and delete transmitted bits as required. The modem also includes a phase rotation means to rotate the phase of received symbols, to counteract the effect of small differences in the timing of the clocks of the two modems corresponding to less than the time taken to transmit a single bit.
The patent describes the activation of bit addition and deletion means in both the transmitting and receiving paths of one of the two communicating modems, to align the symbols transmitted by both modems. Any corresponding equipment that may be present in the other of the two modems is deactivated.
However, the addition or removal of bits causes symbol timing jitter. This causes a variation in the interference structure that can be very large and abrupt which leads to spurious errors on the transmission link.